1. Field of the Invention
This invention relates to a process for treating dewaxing catalysts.
Catalytic dewaxing processes not only remove the waxy components of hydrocarbon feedstocks, but also convert these components into other materials of higher value. Catalytic dewaxing processes achieve this end by selectively cracking long chain n-paraffins to produce low molecular weight products which may be removed by distillation.
Catalysts usually employed in a dewaxing reactor have a pore size which admit the straight chain n-paraffins but exclude more highly branched materials, cycloaliphatics and aromatics. The catalyst usually employed are the zeolites which include the intermediate pore size zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35, ZSM-38 and ZSM-48. These zeolites may also contain a hydrogenation-dehydrogenation component, e.g., a noble metal such as platinum, or palladium, a base metal such as nickel, tungsten, etc. or base metals.
Typically, the catalytic dewaxing reactor is operated at a start of cycle temperature of 540.degree. to about 580.degree. F. (232.degree.-304.degree. C.). The operating temperature is increased by about 2.degree. to about 10.degree. F. per day--depending on feed, catalyst and space velocity--to compensate for decreased catalyst activity. By continuous temperature increases a lube having a specific pour point may be continually produced. Temperatures are increased to an end-of-cycle ceiling temperature of between 655.degree. and about 695.degree. F. (346.degree.-368.degree. C.), usually about 675.degree. F. (357.degree. C.). At the end of the cycle, usually about 10 days, the reactor must be shut down to regenerate the catalyst.
Catalytic dewaxing catalyst reactivation or regeneration is expensive and an initial regeneration or regenerations is usually accomplished by high temperature H.sub.2 regeneration conducted between 900.degree. and 980.degree. F. (482.degree.-526.degree. C.). Hydrogen regeneration generally removes soft deposits of coke. Hydrogen regeneration, however does not always completely restore the original level of activity of the catalyst. For example, it has been observed that following hydrogen regeneration of an HZSM-5 catalyst, the cycle length for a catalytic dewaxing operation is substantially less than the original cycle length. The number of days the catalyst can remain on stream decreases from cycle to cycle and eventually continued reactivation becomes impractical. This loss of activity is primarily caused by a hard carbonaceous residue containing nitrogen, sulfur and oxygen heteroatoms left on the catalyst which hydrogen regeneration does not remove. After a predetermined level of deactivation, oxygen regeneration is employed to burn this hard coke residue off the catalyst and achieve activity resembling that of a fresh or original catalyst. Although oxygen regeneration restores catalyst activity, such treatments are expensive, and the high temperature required for regeneration can result in catalyst sintering. Catalyst regeneration is described in more detail in U.S. Pat. Nos. 3,904,510; 3,986,982; and 3,418,256.
While air (oxygen) regeneration can be effective to rid the catalyst of hydrocarbon residues, a decrease in cycle length has also been observed. When the catalyst contains a metal component, e.g., a hydrogenation-dehydrogenation noble metal such as platinum or palladium and/or a base metal such as nickel, air regeneration can result in still other problems such as metal sintering and agglomeration.
The present invention relates to pretreating a dewaxing catalyst to increase the original cycle length, subsequent cycle lengths, and the useful life of such a catalyst by passing a low molecular weight aromatic hydrocarbon over a dewaxing catalyst at a temperature greater than 800.degree. F. for a time sufficient to deposit on the catalyst between 2 and 30% of coke, by weight of the catalyst. The pretreatment may be conducted in the presence of hydrogen gas.
It is known to coke large pore zeolites. For example, U.S. Pat. No. 4,541,919 to LaPierre et al, which is incorporated herein by reference, discloses the coking of zeolites having Constraint Index of less than 1. Such zeolites include zeolite X, zeolite Y, ZSM-3, ZSM-4 and ZSM-20, and Zeolite Beta. Coking such large pore zeolites restricts the pore size of the zeolite. Coking large pore zeolites allows such a catalyst to simulate a shape-selective zeolite such as ZSM-5. Sources of coke used on large pore zeolites per LaPierre et al include relatively light hydrocarbons, such as n-butane through n-hexane and highly aromatic cycle oil. Such cycle oil is characterized by the presence of dicyclic aromatics. La Pierre et al disclose or suggest nothing about the process of increasing the cycle life or useful life of intermediate pore or even larger size dewaxing catalysts. LaPierre et al is merely concerned with reducing larger pore sizes.
U.S. Pat. No. 4,231,899 to Chen et al, which is incorporated herein by reference, teaches the deposition of coke within the pores of a ZSM-5 catalyst used in a catalytic reaction in which steam may be present as a component of the feed or as a reaction product. Chen et al suggest that the active sites on the catalyst are protected by the coke. Compounds used by Chen to deposit coke within the pores of ZSM-5 include unsaturated hydrocarbons such as olefins, diolefins, dicyclic aromatics, picoline N-oxide and tripropyl N-oxide. Chen et al do not teach or suggest coking intermediate pore zeolites for use in catalytic dewaxing processes to extend cycle length and catalyst life.
European Patent Application No. 134,076, filed June 6, 1984 by Chang et al which is incorporated by reference herein, relates to the use of coked ZSM-5 catalyst for use in disproportionating toluene to para-xylene. Chang et al disclose that coking enhances the diffusability of the catalyst and they have discovered that liquid hourly space velocity can be reduced using such a coked catalyst.